The cervix is the portion of the uterus connecting the uterus to the vagina. The cervix is cylindrical or conical in shape, approximately one inch in length, and having a cervical canal passing through it with an external os opening to the vaginal cavity and internal os opening to the uterine cavity. During labor and delivery, the cervical canal is the channel through which the baby passes from the uterine cavity into the vaginal cavity. During labor, the position (station) of the cervix rotates from posterior to anterior
During labor, in response to coordinated uterine contractions and pressure created by the descending fetal head, the length of the cervix shortens and the cervical walls thin in a process known as “effacement”, and the cervix opens further or dilates. Effacement can be quantified in percentage, from 0% (no change) to 100% (completely thinned). Cervical dilation can be quantified as the diameter of the cervical opening, e.g., in centimeters ranging from zero (0) to ten (10) centimeters. When the cervix dilates to ten (10) centimeters or greater, the cervical dilation can be deemed complete, and the patient can be encouraged to push the baby out. Before effacement and complete dilation, patients are encouraged not to push due to the risk of injury to both mother and baby. Effacement and dilation are critical indicators of the progress, or lack of progress, of labor. The degree and rate of effacement and dilation are monitored periodically during the first stage of labor. Slow or inadequate cervical dilation may indicate the need for administering a cervical ripening drug or applying a cervical dilating instruments or the need for surgical delivery.
A digital palpation is currently the standard procedure clinicians (physician, nurse, mid-wife, etc.) use to measure the cervical diameter. In digital examination, the clinician inserts a gloved hand into the vagina and uses the middle and index fingers to palpate or probe the cervix and external cervical os. The fingertips palpate and locate the external cervical os and are then spread until the fingertips contact opposing walls of the cervix. The distance between the spread fingertips corresponds to the cervical diameter. Using the digital palpation approach, the degree of dilation of the cervical os is estimated without any means to confirm visually the spacing between the index and middle fingers while situated within the cervical os.
During the course of labor in a patient, one or more clinicians perform, on average, ten digital examinations. However, digital examination provides only intermittent data for assessment of labor progression. Furthermore, the accuracy of digital examination is very subjective and may depend upon many factors, including the experience, judgment, and the size of the clinician's fingers, and error caused by the stretching of the cervix by the clinician's fingers. Although an individual clinician may achieve acceptable repeatability and accuracy using this method, it is normal to see a one (1) centimeter error or variation in measurement among serial measurements by the same clinician. If different clinicians examine the same patient during the course of labor, the inaccuracy of cervical dilation measurements increases due to inter-clinician variability.
Inaccurate or inconsistent measurements of cervical dilation may hinder the early detection of dysfunctional labor or delivery complications. Furthermore, despite the use of gloves, digital examination increases the risk of infection of the fetal membranes (chorioamnionitis), the lining and/or muscle of the uterus (endomyometritis), or of the infant (neonatal sepsis). This risk increases significantly after the fetal membranes have ruptured, and the risk of infection correlates to the number of digital exams. For this reason, it is preferable to minimize the number of digital exams, particularly after the fetal membranes have ruptured. Other disadvantages of digital examination measurements to determine cervical dilation include the inability to monitor dilation continuously, the procedure may dislodge fetal or uterine monitors, and the procedure is embarrassing and causes even more discomfort to the mother who is already experiencing significant pain due to labor.
Various mechanical and electrical systems have been devised to measure cervical dilation. See, e.g., Cervimetry: A Review of Methods for Measuring Cervical Dilation During Labor, Obstetrics & Gynecology Survey, Vol. 55(5): 312-320 (2000); see also, e.g., Sharf Y, Farine D. et al., Continuous Monitoring of Cervical Dilation and Fetal Head Station During Labor, Medical Engineering & Physics 29: 61-71 (2007). See also, e.g., the following U.S. patent Nos. and U.S. patent application Publication Nos.: U.S. Pat. Nos. 2,924,220; 3,768,459; 4,141,345; 4,207,902; 4,245,656; 4,476,871; 4,611,603; 4,682,609; 4,719,925; 5,222,485; 5,450,857; 5,658,295; 5,713,371; 5,935,061; 6,039,701; 6,066,104; 6,200,279; 6,270,458; 6,383,137; 6,419,646; 6,423,000; 6,423,016; 6,524,259; 6,540,977; 6,669,653; 6,802,917; 6,966,881; 6,994,678; 7,150,108; 7,207,941; US 2005/0049509; US 2006/0020230; US 2007/0156067; US 2007/0156068; US 2007/0179410; US 2007/0179410; US 2007/0213640; US 2008/0021350; and also PCT Patent Application Publications WO 1987/03189; WO 2000/051494; WO 2004/098375; WO 2004/00373.